[otbn] Remove special case in Reg-Reg ALU decode
[otbn] Remove special case in Reg-Reg ALU decode

This was copied across from the Ibex decoder, where it handles
instructions like CMIX, CMOV, FSL and FSR. Here, we don't have any of
that nonsense :-)

Signed-off-by: Rupert Swarbrick <290093e2ad2515ed2f14d749293c8cf8e02ca274@lowrisc.org>

lowRISC/opentitan,lowRISC/opentitan,lowRISC/opentitan,lowRISC/opentitan,lowRISC/opentitan,lowRISC/opentitan